Visual and vestibular sensory cues interact to support self-movement perception and spatial orientation. We hypothesize that MSTd neurons combine visual and vestibular signals that indicate the same direction of self-movement to enhance heading perception. We further hypothesize that superimposed stimuli indicating different headings evoke neuronal responses that contribute to detecting independently moving objects. Our previous work suggests that neurons in dorsal medial superior temporal cortex (MSTd) access both visual and vestibular signals to support an integrated representation of self-movement heading direction. In addition, we have found that: 1. Responses to object motion are enhanced when superimposed on optic flow that indicates a different heading direction, implying that the object is moving independently of the observer. 2. Responses to real movement can be enhanced when combined with optic flow indicating a different heading direction, implying that the head and visual axes are not aligned in some neuron's, hi the proposed studies we will determine if MSTd combines multi-sensory and gaze signals to support veridical heading perception and detect independent visual object motion during observer self-movement. In three Specific Aims we will study cue interactions between: 1) optic flow and object motion, 2) optic flow and real movement, and 3) object motion and real movement. In all three Aims we will monitor neuronal activity and heading perception focusing on: changes in the relative directions of stimuli cueing independent visual motion, and the effects of gaze angle shifts that might alter the reference frame for observer and object motion perception. All three Aims use the same data analyses, but the interpretation of the results is unique to each, especially for visual-visual (Aim #1) and visual-vestibular (Aims 2 &3) effects. Our objective is to determine how cortical neurons combine sensory cues about self-movement both to estimate the observer's heading direction and to detect potentially threatening independent object motion. These experiments will elucidate the neuronal mechanisms of visuospatial disorientation that we have linked to getting lost and risks of collisions in healthy older adults and patients with Alzheimer's disease.